South Jersey Skies: Static from the sky

Have you noticed some static in your television picture the last couple of days? Or some unusual noise in your long-distance telephone calls? If so, you can probably blame the Sun.

Many of these kinds of transmissions are relayed by communications satellites (comsats). They look different than other satellites in our sky. Most satellites can be seen moving along a path in the sky, usually taking a few minutes to go from horizon to horizon.

But communications satellites are different: they don't appear to move. The Echostar 1 satellite, for example, is due south and 43 degrees above the southern horizon, and will always be there. (Disclaimer: these satellites are actually too far away to see with unassisted vision).

This doesn't mean Echostar is magically nailed down to a fixed position in space. It is still in orbit around the Earth.

But at its special altitude of 22,500 miles above the surface, a so-called geosynchronous satellite takes exactly 24 hours to circle the Earth. That's also how long it takes us to circle around the Earth's center, as our planet rotates. So the satellite appears to stand motionless in our sky.

Everything else does move. Most celestial objects, including the Sun, rise somewhere along the eastern horizon, trek across the sky, and set somewhere along the western half of the horizon.

But the Sun does something in addition to the normal east-to-west motion: it also drifts up and down as the year goes on. It makes a high daily arc in the summer, but a much lower one in the winter (which is why it's cold in the winter).

This happens because the Earth is tilted by 23.5 degrees in space. In the summer, our Northern Hemisphere is pointed more toward the Sun than in the winter.

As a result, for part of the year the Sun appears to pass above all the communications satellites as it follows its daily path; the rest of the time it goes below them. Near the equinoxes, the Sun actually goes directly behind each of them.

The strong radio emissions from our home star then swamp the signal from the satellite, much as the roar of a crowd at a party can make it difficult to hear someone next to you who is talking quietly. You can lose the signal altogether for a time.

Since they're spaced out around the Earth, this happens at a different time of day for each satellite. One channel will suffer interference, or even an outage, in the morning; another might get hit in the afternoon.

You might think this should happen on the equinox days, March 19 or 20 and Sept. 22 or 23. That would be the case if we lived on the equator.

But since we live at 40 degrees North, and look at them from a different direction than do equator-dwellers, the comsats are all shifted about six degrees south in our sky. The solar disturbance dates are thereby moved away from the equinox dates by several days, the number depending on one's latitude.

For our latitude, the Sun passes directly behind the satellites around March 4 every spring, and Oct. 9 every fall. The disturbance is strongest on those days, but can also be heard and seen to a lesser degree for a few days before and after. Fortunately, the daily sweep of the Sun from east to west allows it to interfere with any one satellite for no more than 30 minutes each day.

So don't bother calling your television supplier when your favorite program goes off the air this weekend. Just switch channels, and hope that it allows you to use a different satellite.

---

Next Sunday, March 11, is the final performance of “Stars: The Powerhouses of the Universe” at Edelman Planetarium. A new show, “The Search for Life,” narrated by Harrison Ford, begins on March 18. More information is available at www.rowan.edu/planetarium/.